Photosensitive device



Feb. 20, 1951 3, BROWN 2,542,115

PHOTOSENSITIVE DEVICE Filed Jan. 50, 1948 INVENTOR FAY 0. BROWN ATTO RN EY Patented Feb. 20, 1951 UNITED STATES PATENT OFFICE 9 Claims.

Still another object of my invention is to produce such a device having a light sensitive area so small that it can be incorporated into a multiple light sensitive apparatus of relatively small over-all dimensions and each light sensitive area of the multiple unit having its respective or independent circuit. The definition of smallness of dimensions may be determined from the fact that I may use a light sensitive semi-conductor as small as .1 to 1 mm. in diameter.

Still another object of 'my invention is to utilize materials that embody high absorption of radiation, controlled work function at the electrode contacts, and controlled dissipation of the absorbed energy.

A further object of my invention is to utilize for my improved photo-sensitive device a semiconductor having a very satisfactory work function at the contact surfaces, high absorption constant extending into the visible and infra-red spectrum, and said semi-conductor being quite rigid and having a hard surface, also being a good conductor of heat so that the maximum heat may reach the electrode contacts.

Another object of my invention is to provide means for stabilizing the resistance at the contacts, while at the same time easing the tension on the electrodes thus avoiding breakage of the latter not withstanding their very minute diameter.

Still another object of my invention is to provide such a photo-sensitive device which will be sensitive to temperature change or radiation at temperature as high as 1000 deg. C.

These and other objects of my invention will be more clearly understood by referring more specifically to the accompanying specification and drawings wherein Fig. l is a view in perspective of one of my improved photo-sensitive devices for use in a single circuit; Fig. 2 is a longitudinal cross-section of Fig. 1; Fig. 3 is a View of a typical size of my improved semi-conductor; Fig. 4 is a view of the hexagonal crystal shown in magnification of Fig. 3, and Fig. 5 is a view in diagrammatic form of an assembly comprised of a multiple of light sensitive circuits of my invention.

In the drawings like numerals refer to like parts, and l represents a suitably insulated tube enclosing the parts of my improved device. My improved form of semi-conductor is shown as a hexagonal crystal 2, of silicon or germanium and 3 and 3' represent electrodes or conducting wires of very small diameter, preferably tungsten, attached to each end of the crystal 2. Any suitable means may be employed to attach these electrodes to the crystal, but as one of the objects of my invention is to apply pressure on the electrodes at their areas of contact with the crystal I prefer to tie them to the crystal with strong and slender binding wires, such, for example, as wires of tungsten, as small as /1000" diameter. The electrode wires pass through stoppers or bushings of any suitable insulating material 4, A at each end' of the tube I. In case it is desired to evacuate the tube the latter may be sealed at the ends by vacuum seals by methods well known in the art; or for high temperature work the tube may be filled with an inert gas such as argon. If desired a radiation window 5 may be provided in the tube in front of the crystal 2; and if further desired a sleeve 6 may be used to slide 7 over the window, the sleeve having an opening Preferably this spring is located between one end of the crystal and the inner end of the insulating bushings 401' 6' as the case may be. Another function of this spring is to stabilize the resistance at the contacts. A conducting wire Ill loops the spring 2. The binding wires H are shown at each end of the crystal 2.

Referring to Fig. 5 the envelope or box I may be made of any suitable material, such as the well known plastics, for insulating purposes; or, if made of a non-insulating material the box may be insulated. In the box are assembled a multiplicity of the circuits of my invention, each circuit having its own crystal 2 and spring 9. These crystals are preferably staggered in relation to each other as illustrated, and the circuits may be either in the same or in different planes. Such an assembled apparatus may be used in television or any other application for which the invention is adapted, and wherein it is desired to convey current produced by light radiation to corresponding circuits in another complementary apparatus. It is believed, for purposes of simplification, to be unnecessary to illustrate the structural means in detail for carrying out this multiple assembly unit, as such means are well within the skill of any mechanic familiar with the general art.

Everything else being equal the usefulness of a semi-conductor is to be measured by the amount of change of current that accrues from a given amount of radiation. These semi-conductors that are highly sensitive to radiation are usual- 1y referred to as possessing photo-conductivity.

I have chosen as my preferred semi-conductor the hexagonal crystal 2 of silicon shown in the drawing because selenium (which has heretofore been used in the art to a large extent) has a highly selective absorbing power and in a narrow range. Silicon crystals, on the other hand, are almost ideal for certain uses as they have a hard surface and high mechanical rigidity. The absorbing power of silicon is high, ranging from the ultra-violet far into the infra-red. In silicon the absorbed energy travels at three hundred meters per second. Crystalline silicon maintains its basic properties in vacuum even above 1000 deg. C., whereas selenium melts at 217 deg. C. Silicon crystal with tungsten or platinum electrodes when mounted in a neutral atmosphere, such as argon (to prevent evaporation) is sensitive to temperature changes or to radiation at temperature as high as 1000 C. The resistance of a silicon crystal may vary more than a million to one between 1000 deg. C and deg. C. By varying the mechanical pressure the resistance may vary 1000 to 1. When exposed to light with low mechanical pressure the ratio of resistance in th dark to that in the light may often be greater than 100 to 1. Whereas under high mechanical pressure the ratiomay be very low, nevertheless the actual change of current measured in micrc-amperes per volt is greatly increased by pressure. As it is usually change of current rather than ratio of resistance that determines the effectiveness of a light unit in a practical device it will be seen that the factor of high mechanical pressure onsuch a semi-conductor becomes very important.

Referring to Figs. 3 and 4, the hexagonal silicon crystal 2 in these figures is assumed to be about .2 mm. diameter and about 5 mm. in length. In the event that such a crystal is assembled in the photo-sensitive device of Figs. 1 and 5 and with the electrodes contacting with the ends of the crystal at the junctions of the ends of the conductors 3 and 3 and the crystal, under low mechanical pressure and with one volt across the electrodes the resistance at the contacts may be greater than 1,000,000 ohms at room temperature, Whereas the resistance through the entire length of the crystal may be only 1000 ohms. If high pressure is applied on the electrode contaste the contact resistance may be reduced to a value much less than 1 000 ohms of the crystal itself. Thus, by application of pressure the entire resistance may be reduced about one thousand times. When radiation is absorbed and the crystal is between 0 deg; C and 100 deg. C. the

change of resistance occurs almost entirely at the electrode contacts. At deg. C., the resistance change occurs about equally at the contacts and in the body of the crystal whereas above 200 deg. C. the effective change of resistance occurs almost entirely in the body of the crystal.

In hexagonal silicon crystals between room temperature and 400 deg. K variation of conductivity is due to varying number of electrons having energy greater than .23 volt. With minute electrode contact area this value may vary because of the nature of the surface state with unbalanced electric field. Between 500 deg. K and 1300 deg. K the variation is largely due to the number of electrons having energy greater than 0.8 volt.

By pressure only at the electrodes the low value work function can be reduced practically to zero, while the conductivity through the crystal lattices remains unchanged.

The proportional conductivity change by temperature (or radiation) is reduced by increased pressure, but within limits the absolute change of conductivity is increased by pressure.

The greater the pressure on the electrodes the greater is the change of current by temperature change caused by radiation. This is an important feature of my invention. However, as the contact resistance approaches the value of the resistance in the crystal itself, the measured effective change diminishes, because the crystal resistance is in series with the contact resistance. Thus, for an effective light sensitive device working under 100 deg. C. the contact resistance should be several times the value of the crystal or body resistance, which two are necessarily in series. Likewise the change of current with voltage (illumination constant) should increase proportionately to the voltage applied where it not for two other effects. First the increased voltage deforms the electric field at the electrodes; also, the increased voltage produces proportionately more heat at the contacts. The result of these two effects is to lessen the apparent sensitiveness with high voltage.

On the basis of the foregoing considerations I have devised a new and useful highly light sensitive unit of very small dimensions to be used in either of two ways. First, with low voltage across the crystal continuously (crystal 0.1 to 0.5 mm. cross section) there is a change of current of 20 micro amps, per volt sq. mm. area of exposed surface of crystal when the radiation is that received from a 100 watt lamp at 1 meter distance. The recovery of the crystal takes place in a few micro sec.

Afine tungsten wire 1/1000 is tied around each end of the crystal. The pressure at the electrodes is determined by the stretch of a, coiled spring on one end of the wires. The tension is adjusted until the desired sensitivity is attained. With 100 watt lamp at 1 meter distance the energy received is less than 25 micro watt pe mm. square.

If R equals 10,000 ohms and 1 Volt, the heating by current is 100 micro watts. The energy that is received by radiation or that is produced in the crystal by the current must escape either by radiation or by conduction through ai and wire electrodes.

The advantage of small wire electrodes is twofold. The escape of heat is reduced comparable to the escape by radiation and gas conduction and the length of the wire largely controls the dissipation of absorbed energy. Second, the small size of wire enables the production of a high intensive pressure at the electrical contact with very small force on the wire. Moreover the small size of wire permits a maximum of radiation to impinge on the crystal. It has three advantages over any other known electrode contact, including sharp point contact, vis., the energy lost by conduction is less and under better control, the resistance is more stable, partly because of the spring tension, and, very important a large number of lattices or grid circuits can be encompassed in a small area, such as might be desirable in some television or other light control equipment. A single circuit or a multiple lattice or grid circuit can all be encased in vacuum to reduce energy lost by conduction, or in certain high temperature applications the sealed enclosure for the device may be filled with an inert atmosphere, such as argon, for example.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A light sensitive apparatus comprising a photo-sensitive element selected from the group consisting of silicon and germanium, and having an area of exposure to light from one-tenth to one square millimeter, slender electrodes in electrically conductive relation with the element, means comprising binding wires of low heat capacity such as tungsten in pressure-engagement with the electrodes at the areas of contact of the latter with the element to reduce dissipation of heat produced by radiation impinging on the crystal, and means comprising a tension device cooperating with the electrodes, binding wires and element for stabilizing the resistance at the junctions of the electrodes and element, and minimizing the liability of breakage of the electrodes.

2. A light-sensitive apparatus housed in an envelope comprising a photo-sensitive semi-conducting element of silicon, electrodes in physical contact with said element and extending out of the envelope, means in the envelope comprising binding wires securing the electrodes in pressure contact with the element, and tension means also in the envelope and attached to an electrode adjusting the pressure of the electrodes on the crystal proportionately to the variations in current in the circuit due to temperature rise at the junctions of the electrodes and crystal and the dise tortion of the electric field at said junctions.

3. A light-sensitive apparatus housed in an envelope comprising a photosensitive semi-conducting element of silicon having an area of exposure to light from one-tenth to one square millimeter, electrodes minute in diameter in physical contact with said element and extending out of the envelope, pressure means in the envelope cooperating with the electrodes and crystal for exerting pressure on the electrodes against the crystal, and a coiled spring attached to one of the electrodes for exerting a tension on the pressure means to increase the pressure of the latter in proportion to the increase in temperature at the junctions of contact between the electrodes and the crystal and also easing tension on the electrodes.

4. A light-sensitive apparatus comprising a photosensitive hexagonal semi-conducting crystal having an area of exposure to light from onetenth to one square millimeter, electrodes bound to each end of the crystal by wire having a diameter of the order of one-thousandth of an inch to effect pressure contact of the electrodes with the crystal, and a coiled spring attached to one '6 of. theelectrodessaid spring stretched to ease the tension on the electrodes and stabilize the resistance at the contacts.

5. A light-sensitve apparatus comprising a hexagonal crystal of silicon as a semiconductor and slender wire electrodes of the order of 1/1000" in diameter in contact therewith the dimensions of the silicon crystal being from onetenth to one square millimeter in terms of light exposure area; means for applying pressure on the electrodes against the crystal at the areas of contact between the electrodes and crystal, and a stretched coil spring between one end of the crystal and an end of one of the electrodes and attached to the latter.

6. The method of controlling variations of current in light sensitive apparatus comprising a semiconducting element selected from the group consisting of silicon and germanium and having slender electrodes in electrically conducting contact with each end of said element consisting in tying the electrodes to the element with minute binding wires of low heat capacity such as tungsten, to apply pressure on the electrodes and limit the quantity and rate of flow of heat from the element by conduction, comparable to that lost by radiation and convection, stabilizing the resistance at the contacts, and increasing the current change in the circuit external of the element notwithstanding changes due to temperature rise and variations in electric field at the above mentioned contact areas.

7. The method of controlling variations of current in light-sensitive apparatus comprising a hexagonal crystal selected from the group consisting of silicon and germanium and minute electrodes of the order of one thousandth of an inch in diameter in electrically conductive relation with each end of the crystal and whereby the maximum radiation may impinge on the crystal, consisting in tying the electrodes to the crystal at each end of the latter whereby a predetermined pressure between electrodes and crystal may be exerted, stabilizing the resistance at the contact areas, dissipating the heat produced by radiation impinging on the crystal comparable to the escape by radiation and gas conduction, and controlling the said dissipation of heat by adjusting the length of the minute electrodes whereby the current change in the circuit external of the crystal may be increased.

8. The method of controlling variations of current in light-sensitive apparatus comprising a hexagonal crystal of silicon and minute electrodes having contact areas with each end of the crystal, consisting in applying pressure at the electrodes at said contact areas, stabilizing the resistance at said areas, subjecting the electrodes to tension, adjusting said tension to minimize the breakage of the minute electrodes and dissipating the heat produced by radiation impinging on the crystal comparable to that lost in radiation and gas conduction whereby current changes in the circuit external of the crystal is increased, notwithstanding changes due to temperature rise and variations in electric field at the above named contact areas.

9. A light sensitive apparatus comprising a photo-sensitive element having an area of exposure to light from one-tenth to one square millimeter, electrodes bound to each end of the element by wires having a low heat capacity such as tungsten and a diameter of the order of onethousandth of an inch to effect pressure contact 7 of the electrodes with the element, and resilient means such as a coiled. spring attached to one end of the electrodes intermediate an end of the element and the other end of the electrode.

FAY C. BROWN;

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 853,492 Beck May 14, 1907 1,381,474 Kunz June 14, 1921 1,739,256 Pender et a1 -1--- Dec. 10, 1929 2,395,759 Priessman Feb. 26, 1946 2,402,662 Ohl June 25, 1946 Power of Metallic Germanium, May, 1922, pages 447-455 of Physical Review, volume 19.

Merritt: On Contact Rectification by Metallic 10 Germanium, volume 11, 1925, pages 743-748 of Proceedings National Academy of Science.

Cornelius: Germanium Crystal Diodes, February, 1946, pages 118-123 of Electronics.

Teal et a1.: A New Bridge Photo-Cell Employ- 15 ing a Photo-Conductive Effect in Silicon," November, 1946, pages 879-883 of Journal of Applied Physics. 

1. A LIGHT SENSITVIE APPARATUS COMPRISING A PHOTO-SENSITIVE ELEMENT SELECTED FROM THE GROUP CONSISTING OF SILICON AND GERANIUM, AND HAVING AN AREA OF EXPOSURE TO LIGHT FROM ONE-TENTH TO ONE SQUARE MILLIMETER, SLENDER ELECTRODES IN ELSCTRICALLY CONDUCTIVE RELATION WITH THE ELEMENT, MEANS COMPRISING BINDING WIRES OF LOW HEAT ACPACITY SUCH AS TUNGSTEN IN PRESSURE-ENGAGEMENT WITH THE ELECTODES AT THE AREA OF CONTACT OF THE LATTER WITH THE ELEMENT TO REDUCE DISSIPATION OF HEAT PRODUCED BY RADITION IMPINGING ON THE CRYSTAL, AND MEANS COMPRISING A TENSION DEVICE COOPERATING WITH THE ELECTRODES, BINDING WIRES AND ELEMENT FOR STABILIZING THE RESISTANCE AT THE JUNCTIONS OF THE ELECTRODES AND ELEMENT, AND MINIMIZING THE LIABILITY OF BREAKAGE OF THE ELECTRODES. 